Spectroscopic study of the I₂ formation from the photolysis of iodomethanes (CHI₃, CH₂I₂, CH₃I, and CH₂ICl) at different wavelengths

Emission spectra following the photolysis of iodomethanes (CHI₃, CH₂I₂, CH₃I, and CH₂ICl) at 266 nm were recorded in a slow flow cell. In addition to emission from the electronically excited species including CH (A²Δ, B ²Σ^-, and C²Σ⁺), C ₂ (d³Π_g), and atomic iodine ( ⁴P^o), a series of emission bands was observed in the 12 000-19 000 cm^-1 region. The dominant structure of these emission bands was verified as the I₂ B³Π⁺ _0,u-X¹Σ⁺_g emission at the 532 nm excitation, and the observed I₂ was formed from collisions between iodine atoms generated from the C-I bond dissociation in these iodomethanes. The I₂ emission spectra following the photolysis of CH ₂I₂ at different wavelengths were acquired, and the threshold energy for the first C-I bond cleavage was determined to be 208 ± 1 kJ mol^-1. We also obtained the emission spectra of pure I₂ at several visible excitation wavelengths for comparison with those from the photolysis of iodomethanes, and a least-squares global fit of the observed I₂ emission bands yields more accurate anharmonicity parameters for the vibrational structure in the I₂ B-X transition.